Set size and long-term memory/lexical effects in immediate serial recall: Testing the impurity principle

The impurity principle (Surprenant & Neath, 2009b) states that because memory is fundamentally reconstructive, tasks and processes are not pure. This principle is based on a long line of research showing the effects of one memory system or process on another. Although the principle is widely accepted, many researchers appear hesitant to endorse it in extreme edge cases. One such case involves the effects of long-term memory and lexical factors when a small, closed set of items is used. According to this view, because the subject knows the set of items, there will be no effect of item information. In contrast, the impurity principle predicts that such effects can still be observed, because immediate serial recall with a small closed set of items is not a pure test of order information. Four experiments tested this edge case. In Experiments 1 and 2, we found concreteness effects when item uncertainty was minimized in both within-subjects (Exp. 1) and between-subjects (Exp. 2) designs. In Experiments 3 and 4, we found frequency effects when item uncertainty was minimized in both within-subjects (Exp. 3) and between-subjects (Exp. 4) designs. Analyses of intrusion and omission errors indicated that the sets of items had been learned. Analyses by experiment half also confirmed that the effects of concreteness and frequency were observable in the latter stages of the experiments, when there should have been even less doubt about the items. The results support the impurity principle and suggest that hesitation about accepting it in edge cases is unwarranted.